1. Field of the Invention
The present invention relates to a communication scheme for supporting data transfer to a visited site of a mobile terminal which moves across a private network operated by a private address system and an Internet operated by a global address system.
2. Description of the Background Art
A router device is used in connecting between LANs, and plays a role of transferring data packets (datagrams) from one LAN to the other LAN. Each data packet contains a description of its source and final destination network layer addresses (such as IP addresses in the case of IP, for example) in addition to communication data to be transferred, and the router device determines an output interface (port number) and a next transfer target node (a host that is either a router device or a communication terminal) of each data packet according to its address information.
Here, a routing by the router device will be described for an exemplary case of transmitting IP packets from a terminal A to a terminal B on a network.
An IP packet sent out from the terminal A is routed within the Internet according to an IP address written in its packet header, and transmitted up to a subnet to which the terminal B belongs. At that point, whenever a router device within the Internet is passed through, an IP forwarding processing in a sequence of: reception of a datalink layer packet→conversion into an IP packet→processing of an IP header Information, etc. and an output target determination→conversion into a datalink layer packet→transmission of a datalink layer packet, is carried out with respect to an IP packet transmitted from the terminal A. Once it reaches to a router device of the final subnet, a logical address to physical address conversion is carried out by ARP (Address Resolution Protocol) (by obtaining a MAC address from an IP address, for example), and an IP packet is converted into a datalink layer packet by attaching a datalink layer header information such as MAC address, and this datalink layer packet is transmitted to the terminal B.
On the other hand, a technique for accommodating mobile terminals into the Internet-like network has been studied and developed. As one such mobile access technique, a scheme utilizing Mobile IP is known.
Mobile IP deals with the case where a user carrying a mobile terminal communicates while moving over the IP network across a plurality of subnets. In the case of communicating while moving over the IP network where the network address of the mobile terminal changes as the location of the terminal changes, there is a need to provide a scheme for managing a location of the terminal and transferring communication contents properly, and Mobile IP is a scheme that is proposed in order to fulfill that need. Mobile IP is a technique which makes the mobile terminal to appear to the other terminals as if it is connected at a location where it is normally expected to be located (a home address of a home network), regardless of a location at which the mobile terminal is connected on the network. Mobile IP has been promoted to RFC level by IETF in October, 1996.
In the following, the basic operation of Mobile IP will be described with reference to FIG. 1, which shows an exemplary network configuration in which a first network 100-1 and a second network 100-2 are connected through Internet 100-3. Here, each of these networks is assumed to be operated using global addresses.
In Mobile IP, a network (home network) 100-1 to which a mobile terminal (mobile host: MH) 103 is normally connected has a router called home agent (HA) 105 located therein. The mobile terminal 103 is assigned with a home address at the home network, which is managed by the home agent 105. Here, for the sake of explanation, it is assumed that the home address of the mobile terminal 103 is “10.2” and the address of its home agent 105 is “10.1”.
Next, suppose that the mobile terminal 103 moves and makes a connection at a visited site network 100-2. The visited site network 100-2 has a router called foreign agent (FA) 106 located therein. After moving, the mobile terminal 103 receives packets destined to the own node via this foreign agent 106. Here, for the sake of explanation, it is assumed that the IP address of the foreign agent 106 is “20.1”. In this case, the address “20.1” of the foreign agent 106 will be used as the current location address (called care-of address) of the mobile terminal 103.
In order to notify the current location of the own node, the mobile terminal 103 notifies the IP address “20.1” of the current location from the visited site to the home agent 105. Upon receiving this notification, the home agent 105 manages a correspondence between the home address “10.2” and the current location address “20.1” of the mobile terminal 103.
Now, Mobile IP assumes that there exists a correspondent host who does not know that the mobile terminal 103 has moved. This correspondent host (CH) 109 that tries to communicate with the mobile terminal 103 does not know that the mobile terminal 103 has moved so that it transmits packets destined to the home address “10.2”. However, the mobile terminal 103 is currently absent from the home network. Hence, the home agent 105 which manages this mobile terminal 103 intercepts (receives as a proxy) a packet destined to the home address “10.2”, and transfers it by encapsulating it into an IP packet destined to the current location address “20.1” that is registered in advance. The foreign agent 106 that is located at the address “20.1” takes out this packet from the encapsulated packet that is transferred thereto, and delivers it to the visiting mobile terminal 103 at the datalink layer. By such a mechanism, the mobile terminal 103 can receive a packet that would have been received by the own node at its original home network 100-1, even at its visited site.
In the opposite case where the mobile terminal 103 returns a packet to the correspondent host 109 from its current location, an IP packet is sent to the correspondent host 109 directly without going through the home agent 105. At that point, the home address “10.2” is used as a source address to be attached to that packet, rather than the current location (care-of) address “20.1”. In this way, it appears as if the mobile terminal 103 is continually connected to its home network, from the correspondent host 109, so that the influence of the moving can be eliminated entirely.
As described above, in Mobile IP, each node (correspondent host) on the network can access the mobile terminal 103 regardless of a location on the network at which the mobile terminal 103 is connected, by using the home address of the home network 103.
Now, in the Mobile IP scheme, a protocol is designed by assuming only a case where the mobile terminal moves within a single address space. That is, it has been assumed that the current location registration message (which is also referred to as the mobility registration message) from the visited site can reach the home agent of the home network without failure.
However, nowadays, in the case where a large organization makes Internet connections, it is rather rare to have global IP addresses allocated to all the nodes within the organization because of the shortage of IP addresses, and usually private addresses (see RFC 1597) are used for operation within the organization and the address conversion to global addresses are carried out at a time of communications with the external.
Thus, in such a case, Mobile IP can be utilized only within a range where the mobile terminal moves within a single address space. In other words, there has been a problem that Mobile IP cannot support those services in which the mobile terminal is allowed to move across networks operated by mutually different address systems.
FIG. 2 shows an exemplary case where Mobile IP cannot be utilized, which is a network model in which a communication service provider offers a service using mobile terminals that are capable of making Internet accesses. FIG. 2 shows an exemplary network configuration in which a private network 101 operated by a private address system and a global Internet 102-2 operated by a global address system are connected through Internet 102-1 operated by a global address system.
The number of global addresses that are allocated to one communication service provider is limited, so that communications using private addresses will be carried out within the private network 101 operated by the communication service provider. This service can possibly be required to support not only data transfer in the case where the mobile terminal 103 moved from the home network 101a to another subnet 101b and accesses to servers 201 and 202 of the private network 101 by the mobile terminal 103, but also accesses to a server 203 which is generally connected to the global Internet 102-1 from within the private network 101 by using a network address translation device 104 having a network address translation (NAT) function, and direct communications by the mobile terminal 103 that has moved to the global Internet 102-2 in a global region.
However, when the mobile terminal 103 which has its home network in a private region moves into a global region, Mobile IP cannot be used. On the contrary, when the mobile terminal which has its home network in a global region moves into a private region, Mobile IP also cannot be used.
In such a network model, it is not sufficient to realize a simple packet exchange between the private network and the global Internet, and it is also necessary to maintain currently carried out communications by utilizing the properties of Mobile IP even in a case where the mobile terminal that had been connected to the private network shortly before has now moved to the global Internet or vise versa.
In order to realize that, it is necessary to carry out the routing control of communication packets by recognizing the current location to which the mobile terminal has moved, comprehending a relative relationship with respect to a location to which the mobile terminal had been connected shortly before, and carrying out the necessary global to private address translation via the entrance of the private network.
As described above, in general, in order to support the mobile terminal which moves across the private network operated by the communication service provider and the global Internet by utilizing the Mobile IP scheme, it is necessary to provide a scheme for controlling a location to be regarded as the current home network according to the current location and the immediately previous location of the mobile terminal and a way of commanding transfer of packets to the current location with respect to the immediately previous home network, and for realizing the routing control up to the final destination by receiving the transmitted or transferred packet at the border between the private network and the global network and converting it into appropriate format.
Thus, in the case of supporting a service in which the mobile terminal can move across networks operated by mutually different address systems, the conventionally known mobile access technique for Internet-like network such as the usual mobile management by Mobile IP in a single address space is insufficient, and there is a need for a mechanism which can properly account for two types of address spaces such as the private network operated by the private address system managed by the communication service provider and the general global Internet, and realize not only the packet exchange across their border but also the proper routing control of packets up to the current location even when the mobile terminal moves across their border.